Home Forums General Medicine Corona Virus Part II – Tests for Covid 19

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    Today Covid 19 is the disease being discussed everywhere around the world. Doctors and Health Care workers are looking for a quick test to check who is infected and who is not. At the moment hundreds of tests are being developed globally. The Foundation for Innovative New Diagnostics (FIND) in Geneva, that keeps a running global list of COVID-19 tests inform us that, as of 3rd April 2020 nearly 400 tests have been registered.

    Diagnostic tests for COVID-19 typically fall into two broad categories:

    1. Molecular tests or Nucleic acid test of secretions from the nose and throat will detect the virus’s genetic code.
    2. Immunoassay (Blood test) will detect proteins associated with the virus.

    Nucleic acid-based tests are the most sensitive for early detection of infection. The test is called PCR (polymerase chain reaction).
    Covid 19 is an RNA virus. A swab from an individual will collect mucous, saliva, bits of cells and with it viral RNA if it is present. The sample is then processed in the lab. First chemicals are applied to remove everything but the RNA from the virus. The RNA must then be converted to DNA by adding enzymes to transcribe the RNA into DNA. The technique by which this is done is called reverse transcription polymerase chain reaction (RT-PCR).

    The DNA is put into a real-time PCR machine along with another set of chemicals. The machine heats and cools the samples repeatedly in a process that essentially copies the DNA, making thousands of copies of any genetic material in the samples. The whole process is done to amplify any segments of viral genetic code that is present in the sample taken.

    Computer software then indicates the test as being positive for novel coronavirus when the number of copies of DNA exceeds a certain threshold value. Paul Yager, a bioengineer at the University of Washington in Seattle says the PCR is the gold standard in viral diagnostics. He says the sensitivity is so good that it can pick up copies of the virus at an exceedingly early stage. If someone is infected but has no symptoms the test will indicate it is positive.”

    The machines can provide results in a couple of hours and the newest nucleic acid test machines are reported to produce results in minutes.

    What are immunoassays?

    When you get infected two antibodies are involved in fighting the infection. IgM is the first antibody to fight against the viruses whereas IgG reacts later but help in the permanent eradication of antigens. The test for COVID-19 here is based on antigen antibody interaction.

    Rather than analyzing genetic material, these tests detect proteins produced either by the virus itself, or by the body’s immune system in response to the virus. The tests can detect evidence of past or current infections. A sample of blood is taken for the test.

    Viruses are covered in protein structures, called spikes, that are unique to that virus. As the body’s immune system kicks in to fight the infection (antigen), it produces its own unique proteins (antibodies), that attach to the protein structures on the virus. The linkage between antibodies and their respective viral proteins (antigens) is used as a basis for the test. The tests use antibodies and antigens as bait to capture their respective partners.

    A sample—typically blood, plasma, or serum—is used for the test. If the sample already contains antibodies that normally appear a few days after the infection, the antibodies will bind with the antigens used in the test. Likewise, in the early stage of infection when the blood sample contains only the viral antigens, the antigens will bind with the antibodies used in the test. Any binding that takes place indicates a positive result. Chemical markers attached to the DNA release fluorescence when this DNA binding occurs. Flashes of fluorescence that is seen indicates the presence of virus in a sample.

    Immunoassays aren’t as sensitive as nucleic acid tests during the early days of an infection. However they are easier to use and would indicate that the individual became infected or was exposed to the infection a few days (or weeks) before the test. This is the basis for the “Rapid Antibody Test”. The test will be useful for surveillance rather than for diagnosis.

    There are a number of “test kits” that are beginning to appear on the market. In order to avoid false negative and false positive tests, it is most important to use only kits and machines that have been tested and approved by the local health authorities.

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